Earth's magnetic north pole has been shifting enough that Tampa's airport has to repaint the numbers on its runways.Tampa International Airport in Florida has closed its primary runway until Jan. 13 to change the numeric designations at each end, as well as the signage on taxiways leading to the runway. The Tampa Tribune said the runway had been designated 18R/36L, indicating its alignment along the 180-degree approach from the north and the 360-degree approach from the south. Now the numbers are being revised to read 19R/1L (190 degrees and 10 degrees).

Two other runways will be closed later this month for a similar signage change, the Tribune reported. The changes are required by the Federal Aviation Administration, which wants the numeric designations to reflect magnetic-north headings to the nearest 10-degree increment.

For decades, the magnetic north pole has been migrating from Canadian Arctic territory toward Russia. That shift has accelerated in recent years, and current estimates suggest that the pole is moving at almost 40 miles a year. Maps from Natural Resources Canada chart the movement since 1831 and project the trend through 2050. Movements in the magnetic poles are caused by the motion of molten iron at Earth's core, which serves as the planet's magnetic dynamo. Here's how NASA explains the process.

Airports generally change their runway designations every few decades, depending on how the pole shifts shake out numerically. For example, Stansted Airport in the London area renamed its 23/05 runway as 22/04 in July 2009 to reflect the magnetic shift. "It'll roughly be another 56 years before we have to consider changing it again," Trevor Waldock, head of airside operations at Stansted, told the BBC at the time.

This NASA Web page explains that the shift in the magnetic poles, or even a pole reversal, need not be feared. "As far as we know, such a magnetic reversal doesn't cause any harm to life on Earth," NASA says. But a lot of runway numbers might need to be repainted.By Alan Boyle, MSNBC;

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